20 HISTORY OF LIFE ON EARTH...our planet- earth such as (i) our universe (ii) our galaxy- milky way,...

MODULE - 5The Living World

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History of Life on Earth

SCIENCE AND TECHNOLOGY

Notes

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HISTORY OF LIFE ON EARTH

It is a fascinating experience to look-up at the sky on a clear night. Have you notwondered while looking up, when and how our planet earth came into existence?Or how life began and such diverse forms of life that we see around have evolved?These are some of the mysteries that scientists have tried to answer. In this lessonyou shall learn how earth was formed, about theories explaining origin andevolution of life forms on this planet. The story will continue upto evolution ofhumans on earth.

OBJECTIVES

After completing this lesson, you will be able to:

• describe the physical conditions of primitive earth;

• discuss the theory of origin of life (Oparin’s theory) and relate it to thechanging environment on earth;

• become aware of Darwin’s major contributions;

• modify Darwin’s thought to incorporate it in Neo-Darwinism;

• identify the levels of organic evolution;

• list the evolutionary events in the history of life;

• trace stages of human evolution through time.

20.1 PHYSICAL CONDITIONS OF PRIMITIVE EARTH

The physical conditions on primitive earth were not congenial for life. The earthwas extremely hot-a ball of hot gases.


MODULE - 5 History of Life on Earth

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Notes

20.1.1 Solar system and the formation of planet Earth

The universe around us is so enormous that it is difficult even to imagine itsdimensions. In a far corner of the Milky Way Galaxy, (one of the billions ofgalaxies comprising the universe), sits our solar system, like a tiny sand particleon a vast sandy beach. Within this system, Earth, the planet on which we live, isone of the planets revolving around a medium-sized star we call the Sun.

ACTIVITY 20.1

Use any general knowledge book/geography book/science book/book onenvironment or use the internet to obtain a picture of earth and the other planetsin the solar system. Highlight location of earth with a pen and observe it withrespect to the sun.

The whole universe formed probably 12 to 14 billion years ago (one billion = 109

or 1,000,000,000) as a result of a ‘Big Bang’ and subsequent expansion. Our solarsystem came into existence 5-7 GY ago (GY = Giga Year). In its initial stages offormation (4.5 GY) earth was impacted by another planet that caused the spin (that gaveus day and night) and tilt (that gave us seasons) of our planet and also led to the formationof moon. For nearly 700 million years (up to 3.8 GY ago), earth experienced frequentand catastrophic bombardment by meteorites of different sizes.

Gradually earth’s crust solidified although volcanoes kept on spewing out harmfulgases. These gases accumulated and combined to form methane, ammonia andhydrogen cyanide. These three gases, all lethal, along with minor gases likecarbon dioxide and carbon monoxide, formed the atmosphere of the primitiveearth. Thus, the prebiotic (before life arose) atmosphere was so unlike the presentone. Note that there was no oxygen then, a gas so essential for nearly all livingorganisms.

ACTIVITY 20.2

Select 5 friends, each one of you represent one of the 5 stages in the evolution ofour planet- earth such as (i) our universe (ii) our galaxy- milky way, (iii) our solarsystem (iv) planet earth (v) India on this planet. You may dress yourselves in waysthat would convey some important information/details about the “characteristics”that each one of you is going to represent. (You can take the help of internet orbooks or yours elders). Here some graphical representations are given below foryour use. You may enlarge the graphics, select the appropriate one for the parts


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each one of you is playing and display then on your dress. Practice your roles well inthe correct sequence and when ready call your other friends and family members andpresent your show “story of 12-14 billion years.”

At the end you may even arrange for a quiz.

20.2. ORIGIN OF LIFE: WHEN, WHERE AND HOW DIDLIFE BEGIN?

There is a general belief that life on earth must have originated, not before 4.0GYand no later than 3.5 GY ago. A more precise estimate is difficult to come up with, sincethe earliest life, did not leave any evidence in the form of fossils. Some fossils(remainsof living beings that once existed on earth) claimed to be cyanobacteria (blue greenalgae) were found in Australia from rocks dated 3.5 GY. But cyanobacteria are fairlycomplex and advanced and therefore we may assume that life originated much earlierthan 3.5 GY. So, for the present we accept that life originated nearly 3.8 GY ago.

One theory, proposed by the British biologist J. B. S. Haldane and the Russian scientistA. I. Oparin, suggested that life originated in the shallow seas where important organiccompounds (such as amino acids), the building blocks of life, were present in highconcentrations (forming a ‘primordial soup’), thus providing the necessary ingredientsfor emergence of life. But where did these organic molecules come from? Haldane andOparin suggested that in the reducing atmosphere (because of the absence of oxygen)of the primitive earth they could have formed from inorganic substances which werewashed down with torrential rains as earth cooled and formed a ‘primordial soup’ inwhich life originated. Later Stanley Miller and Harold Urey provided experimentalsupport for this hypothesis. Under laboratory conditions they successfully producedamino acids by passing an electric charge (simulating lightning) through a flask containingmethane, ammonia and hydrogen in solution. (Fig. 20.1)



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On the deep sea floor of the oceansthere are sites which have vents ordeep cracks through which extremelyhot dissolved gases and minerals keepbelching out like fountains from theearth’s interior. A special group ofarchebacteria thrive near these ventsas they are adapted to live at hightemperatures exceeding 100°C (andhence their name,hyperthermophiles), and deriveenergy chemosynthetically from thehot gases. Evolutionarily, thesemicroorganisms are very ancient (~3.5GY) and probably among theearliest living organisms on earth.These observations lend support tothe more recent hypothesis that life evolved around such hydrothermal vents in theoceans.

Regardless of where life had begun, how life emerged is still a mystery. Even ifwe assemble all the organic compounds essential for life we simply cannotproduce from them a living organism capable of growing, reproducing and, storing andpassing on a hereditary map to its offspring. How was it possible thenthat life suddenly emerged in a certain ‘primordial soup’ on the earth 3.8 GY ago?Did life arise from that soup of organic compounds in single step or through a fewintermediate stages? Scientists are trying to understand the possible intermediatesteps in the origin of life in the hope that one day in the near future they can producein the laboratory a living form from basic organic molecules.

20.2.1 Diversification of Life

Life on earth started in the form of simplest unicellular (prokaryotic) microorganisms.In course of time these organisms evolved to utilize solar energy through chemicalprocess called photosynthesis. You may recall that oxygen is released during thisprocess. It is through the photosynthetic activity of the earliest autotrophs that oxygenbuilt up gradually in the earth’s atmosphere making it possible for complex heterotrophicorganisms to evolve. For a very long time (nearly 3 GY) after the origin of life, earth hadno life forms other than prokaryotes (cells lacking nucleus) comprising different groupsof bacteria. There were neither plants nor animals. Eukaryotes (cells with nucleus)probably appeared about a billion years ago, but life was mostly in the form of unicellular(single celled) organisms. Then suddenly, about 600 million years ago, in a geologicalperiod called Cambrian, there was a great, almost explosive, diversification of life into

Fig 20.1 Oparin and Haldane’s experiment

Belching: to enie contentsviolently

(‘~’ means approximately)

GY = Gega Year


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multicellular organisms with a variety of body plans and life styles,of all thoseinvertebrates and higher plant groups that you are familiar with. Biologists call this periodthe ‘Cambrian explosion’. (See box I)

BOX- I The geological time scale ERA PERIOD EPOCH AGE

(Millions of years)

MAJOR EVENT

Recent Historic time Quaternary Pleistocene 1.8-0.01 Ice ages; Humans appear Pliocene 5-1.8 Ape-like ancestors of humans Miocene 23-5 Continued radiation of mammals

and angiosperms Oligocene 34-23 Origin of most mammalian orders Eocene 57-34 Angiosperm dominance and

increase in mammalian diversity

Cenozoic

Tertiary

Paleocene 65-57 Major radiation of birds and mammals

Cretaceous 144- 65 Flowering plants appear Dinosaurs extinct at the end

Jurassic 208-144 Dinosaur dominance First birds

Mesozoic

Triassic 245-208 Gymnosperm dominance First dinosaurs and mammals

Permian 286-245 Radiation of reptiles Carboniferous 360-286 Extensive vascular tree forests,

origin of reptiles Devonian 408-360 First amphibians and insects Silurian 438-408 Colonization of land by plants Ordovician 505-438 First vertebrates (jawless fishes)

Paleozoic

Cambrian 544-505 Origin of most invertebrate phyla 700 Origin of animals

1,500 Origin of Eukaryotes 2,500 Oxygenbuild-up in atmosphere 3,500 Origin of life

Precambrian

4,500 Origin of Earth

Fossils, the remains of plants, animals and lower living beings provide evidence for thesequence in which different kinds of living organisms came to exist on earth.

When a fossil is collected, the age of the sedimentary rock in which it is foundis determined and that age is generally taken as the time in earth’s history whenthat particular animal lived. Paleontologists (Scientists who study fossils)areable to reconstruct the history of life on earth from the fossils collected insedimentary rocks of different ages. They clearly show that species and highertaxonomic groups (like angiosperms, insects and birds) evolved gradually. (SeeBox I)



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INTEXT QUESTIONS 20.1

1. When did the earth come into existence?

___________________________________________________________

2. Why did life not exist on primitive earth?

___________________________________________________________

3. What are fossils?

___________________________________________________________

4. What is meant by Cambrain explosion?

___________________________________________________________

5. From the geological time scale, find out the time in million years ago(mya) when:

(i) dinosaurs became extinct ______________

(ii) human evolution began ______________

(iii) flowering plants became dominant on earth ______________

BOX IICHARLES DARWIN (1809- 1882)

Charles Darwin is, like Isaac Newton in Physics, agiant in Biology, whose theory of evolutionrevolutionized our understanding of life and itsdiversification on earth.

Darwin was born in Shrewsbury, England on February9, 1809. He developed a passion for nature from earlychildhood, a trait he probably inherited from his grandfather Erasmus Darwin.His father wanted him to study medicine at Edinburgh but Darwin did not have theaptitude for it. He also did not pursue studies to become a cleric, his father’s secondchoice. Darwin was offered the position of a naturalist on board the ship HMSBeagle which he accepted with excitement and enthusiasm.

The voyage on HMS Beagle was a major turning point in Darwin’s life. Duringthe five years (1831-1836) of its voyage he discovered rare fossils in AndesMountains, collected fascinating animals and plants in Atlantic rain forests ofBrazil and made observations on the geographic variation in the famous


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Darwin’s finches on Galapagos Islands. Darwin gained from all theseexperiences valuable insights and scientific support for the theory of evolutionhe was formulating.

Upon return to England, Darwin started accumulating more scientific materialin support of his theory of evolution through a mechanism that he called naturalselection. Darwin’s famous book on “Origin of Species” was published in 1859.

Darwin died in 1882 at the age of 73. He was given state funeral and was buried inWestminster Abbey next to the grave of Isaac Newton.

Scientists around the world commemorated Darwin in 2009 by celebrating his birthbicentennial and the 150th anniversary of the publication of his famous book on the‘Origin of Species’.

20.3.1 Diversity of Life Resulted from Evolution

When we explore nature, we observe that

1. There is so much diversity of microbes, plants and animals in the biosphereof our planet.

2. Many animals and plants share common features. We humans are similar to rats,horses, elephants and tigers in possessing hair and mammary glands. Further, weshare features like vertebral column with birds, snakes, frogs and fishes. In fact, allliving organisms have so many characteristics in common, including DNA, thehereditary molecule.

3. There is so much variation even among individuals of the same species. You can easilynotice that all your classmates are not alike; they differ from each other in features likeheight, facial expressions and skin pigmentation. Likewise, individuals in a school offish, tomato plants in a vegetable garden, Aedes mosquitoes in a water tank, they allshow variation in some feature or the other.

These three observations lead us to ask important questions. How and why did suchhuge diversity of life forms arise? Were the diverse life forms present from the beginningof earth’s history or did they arise gradually over a period of time? Why do evenremotely related organisms have so many features in common? Is it possible becausethey all came from a single ancestor? Why is there so much variation within any singlespecies?

ACTIVITY 20.3

Collect the pictures of a plant, any animal and a human being from old news paperor magazines. Paste the pictures and note in the table shown below 3 features inwhich they are similar and three features in which they differ.



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Living being Similarity Difference

1. 1.

2. 2.

3. 3.

ACTIVITY 20.4

Note the eye colour, hair colour, earlobes of five of your friends and compare themwith regard to the differences. This will give you an idea of variation which isnecessary for origin and evolution of new species.

Name Eye colour Hair colour Ear lobes

Rohan

Mary

Salim

20.3.2 Darwin’s Theory of Evolution- Salient Points

Darwin made important observations and drew inferences from them, whichhelped him in developing his theory of evolution.

The commonness of many features from anatomical to molecular is a clearindication that all organisms evolved from a common ancestor. Darwin concluded thatliving forms were not created but evolved by descent with modification from ancestralforms going back all the way to the earliest life more than 3.5 billion years ago.

Darwin’s next question that needed an answer was-”What is the mechanism by whichthe origin of species by descent with modification could take place?”

Darwin suggested two very important points with regard to evolution.


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(1) All living beings are related through ancestry.

(2) The mechanism which causes diversification of species from ancestors is ‘NaturalSelection’.

Darwin made four important observations on his travel on the ship HMSBeagle. (See Box II)

1. All organisms tend to produce more offspring than can possibly survive.(e.g. only a few frog’s eggs survive and become frogs).

2. In fact, population numbers tends to remain fairly constant over long periodsof time.

3. Also, organisms in a species show wide variation in characteristics.

4. Some of the variations are inherited, and so passed on to the nextgeneration.

From the above mentioned observations Darwin made these two followingdeductions:

1. Since most offspring do not survive, all organisms must be going through astruggle for survival, being eaten, suffering from disease and competition.The struggle for existence cause large number of individuals to die.

2. The ones who have characteristics that allow them to survive and reproducebetter (i.e. possess most useful adaptations for surviving in the environment) willpass on these characteristics to their offspring. In other words, Nature selectsthe fittest individuals of the population. Natural Selection is the same as the famousphrase “survival of the fittest” coined by Herbert Spencer. Organisms withslightly less survival value will probably perish first, leaving the fittest to pass ontheir genes to the next generation.

To sum up, therefore, the best adapted individuals were selected by nature tosurvive and leave offspring for the next generation. Darwin called this mechanismNatural Selection.

In Darwin’s time it could not be demonstrated that Natural Selection was themechanism by which organisms evolved, but later scientists were able to findsupport for Darwin’s theory in nature as well as in laboratory experiments.

ACTIVITY 20.5

Read carefully Box III and justify why many grand parents tell grandchildren thatthey slept comfortably in the open because there were no mosquitoes and wonderhow mosquitoes have reappeared and it is impossible to sit outside after sun down.



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20.3.3 Neo-Darwinism

Although Darwin talked about heritable variation, he did not know exactly howheritable characters arose and were passed on from one generation to another. Thisis because Darwin was not aware of the principles of genetics developed byMendel a few years before the publication of his book ‘Origin of Species’.Incorporation of Mendelian genetics into Darwinian theory later by evolutionarybiologists led to the emergence of Neo-Darwinism. Further refinements in thelight of advances in population genetics and other areas of biology led to themodern synthetic theory of evolution. It is to Darwin’s credit that his basic theoryof evolution by natural selection finds support even in the most recent developmentsin molecular biology. Read Box III to learn how natural selection acts on variation toproduce new species. Also see figure 20.2

BOX-IIINatural Selection in Action

Although Darwin and many scientists of his time were convinced that naturalselection is the right mechanism by which species evolve, they could not proveit experimentally or otherwise. It was felt that since any visible modificationin a species evolves slowly over a long time, the effects of natural selectioncannot be demonstrated easily. But, now we know that it can certainly bedemonstrated, as you can see from the following examples:

1. Industrial melanism in peppered moth (Biston betularia) (see Fig.20.2)

Fig: 20.2a The typica (t) and carbonaria(c) forms of Biston betularia on the light-

coloured trunk of a birch tree

20.2 b. The typica (t) and carbonaria (c)forms of Biston betularia on the soot-

darkened trunk of a birch tree


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Peppered moth is a common moth in England and it occurs in two varieties-a light-coloured variety called typica and a dark-coloured carbonaria. Whenthese moths rest on the light-coloured trunks of trees, the typica moths getnicely camouflaged and cannot be spotted by birds that catch and eat them. Butthe carbonaria moths being dark against the light background of the bark standout and are be easily located by the birds and captured. Because of this, thecarbonaria moths suffered more predation and therefore always remained invery low numbers in the population. Then the industrial revolution in Englandin the mid 19th century brought in many coal-based industries and the resultingsoot started depositing on the trunks of trees on the countryside. Soon after,scientists noticed that the numbers of typica moths started going downdrastically while those of the carbonaria forms increased.

How did it happen? The soot deposits made the bark black and against the darkbackground, the carbonaria forms now had the advantage of camouflagewhereas the typica moths became more and more vulnerable to bird predation.Consequently, the carbonaria form increased in numbers while typica numbersdecreased. This is natural selection in action. In the industrialized England the darkcarbonaria variety of the moth had the selective advantage because they escapedfrom bird predation more often and lived to leave more offspring for the nextgeneration. Only the best adapted live and leave their genes in the next generation.

2. Evolution of insecticide-resistant mosquitoes

In our desperate efforts to control pest and disease-carrying vector insects, wehave been spraying pesticides like DDT in large and larger quantities, but havenot been able to eliminate them. When we spray a poisonous chemical on apopulation of mosquitoes, surely many of them die. But in any population thereis variation for resistance and a few genetically resistant individuals survive thespraying. They breed and produce resistant offspring. In the next generationthe proportion of pesticide-resistant individuals is higher. As spraying practicecontinues, the entire population becomes resistant in a few generations andthus a genetically distinct variety of mosquito has evolved on whom DDT hasno effect.


1. Who is Charles Darwin? Name his famous book on natural selection.

___________________________________________________________

2. Mention his two major contributions.

___________________________________________________________



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3. What is the function of Natural Selection?

___________________________________________________________

4. What is meant by Neo Darwinism?

___________________________________________________________

5. Name the evolutionary mechanism that causes organisms to evolve.

___________________________________________________________

20.4 LEVELS OF ORGANIC EVOLUTION

With progress in various fields of Biology, the theory of evolution by NaturalSelection became more and more acceptable.

The unit of evolution, in the modern synthetic theory of evolution, is thepopulation. It is the population which evolves and not the individual. Variationoccurs at the genetic level through mutation and sexual reproduction in the “genepool” of the population (gene pool means all the different genes in a populationof individuals). Natural Selection causes greater reproduction of the variantgenes having adaptive advantage.

Evolution at the level of the hereditary material or genes that is the gene pool ofa population is termed microevolution. Populations of a species differ due tomicroevolution. Macroevolution or adaptive radiation is the evolution anddiversification at the level of species and genera. eg Dinosaurs evolved as runners,fliers, swimmers due to macroevolution or adaptative radiation.

20.5 MAJOR EVENTS IN THE HISTORY OF LIFE

As we mentioned earlier, all the different life forms that we see on earth nowevolved only gradually. Radiometric dating of geological (earths’) strata anddetailed study of the fossils found in them help us in reading important‘chapters’ in the history of life on earth since its origin 4.5 billion years ago.Geologists have given names to different periods of this history (see Box I). Youmay recall that microscopic, unicellular prokaryotes were the exclusive lifeforms on earth for nearly three billion years. Dinosaurs ruled the earth for nearly150 million years and became extinct 65 million years ago. Recall from sub-section 20.2.3, the appearance of eukaryotes and the Cambrian explosion. If youcould have traveled 200 million years back in earth’s history, you would havefound neither birds nor flowering plants anywhere on earth! When did wehumans arrive on this planet? Just 2 million years back (see the box ongeological time scale)! If you consider the 24 hours geological clock with the


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origin of life set at midnight, we can say that humans have come on thisplanet just less than a minute ago

20.6 STAGES OF HUMAN EVOLUTION

When human evolution began, forests had dwindled because of glaciation (ice age).Much of the land surface was however, still covered by forests. The common ancestorsof apes and humans had to come down from trees where they lived and walk on theground using all four limbs. Recent molecular studies have shown that from commonancestors, evolution of apes (Chimpanzee, gorilla, gibbon and orangutan) and that ofhumans, diverged about 6 million years ago.

The trends of human evolution are towards (i) bipedal gait or walking on two legs and(ii) acquiring a large brain.

Fossil history reveals that human evolution began approximately 1.5 to 2 million yearsago. Australopithecus is deemed to be the first human like ancestor. Fossils of anaustralopithecine named ‘Lucy’ has been found in African rock deposits. Thereafter,fossils of Homo erectus which walked on two legs, were unearthed from many partsof the world.

Next to evolve was Neanderthal man and Cromagnon man. They were both Homosapiens. Modern man, Homo sapiens sapiens meaning the wise one evolved about50,000 years ago. Since then, biological evolution of humans has perhaps notoccurred. But vast steps of cultural evolution has made humans land on the moon!

Evolution of humans showing ancestor common to apes and humans



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ACTIVITY 20.6

If you have enjoyed doing activity 2 where you and your friends enacted in the story of“12-14 billions years. You can arrange similar show with the heading “origin andevolution of Homo sapiens sapiens.

Some changes is required in graphic.


1. When did human evolution begin?

___________________________________________________________

2. Who is ‘Lucy’?

___________________________________________________________

3. Write the scientific name of Cro-magnon and Neanderthal man

___________________________________________________________

4. With which group of animals do humans share their immediate ancestors.

___________________________________________________________

5. Name the earliest ancestors of modern day humans.

___________________________________________________________

WHAT YOU HAVE LEARNT

• We live on planet earth which is 4-5 billion years old.

• The earth along with other planets, their satellites, the sun, moon, the many galaxiesform the universe.

• A solar system consists of a star in the middle with number of planets orbitingaround it.

• Our planet earth is a part of its solar system and the sun is the star aroundwhich it revolves.

• Age of our earth is about 4.5 billion years.

• In the beginning, the earth was very hot but gradually the surface of earthcooled to form a hard rock.


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• Life originated on earth in the distant past from chemical compounds througha series of chemical changes that occurred in water (chemosynthetic theory)proposed by AI Oparin and Haldane.

• Large molecules like proteins were formed which got together surrounded bymembrane to produce the precursors of primitive cell. Thus unicellularorganisms, came into being.

• It has, however, not been possible to create a cell in the lab.

• In the geological era called Cambrian, there was a great almost explosivediversification of multicellular organisms of different shapes, sizes andfunctions. This is known as Cambrian explosion.

• Evolution is formation of complex organisms through change from simpleancestral types over the course of geological time.

• Darwin’s two major contributions were the idea of :(i) shared ancestry and(ii) natural selection as mechanism of evolution.

• According to Darwin, organisms produce more offspring than can survivebecause environmental resources are limited.

• In the organisms’struggle for existence, those with advantageous (variations)characters survive and reproduce to leave more offspring while the disadvantageousvariants are eliminated. This is known as natural selection.

• With progress in genetics sources of variations were discovered and Darwin’soriginal theory of natural selection was modified as Neo-Darwinism ormodern synthetic theory.

• From the time of origin of earth (4.5 billion years ago) to the present, the entire period isdivided into different eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic.

• Major events of evolution were: Cambrian explosion and advent of mammalsin the Cretaceous era.

• Human evolution began approximately 1.5 million years ago and main stageswere Australopithecus, Homo erectus and Homo sapiens.

TERMINAL EXERCISES

1. What were the primitive conditions on earth. Tell your friend/cousin/colleague. Then ask your friend to name the gas which was absent withoutwhich today no life can exist.

2. What are the main points of Oparins’theory of Origin of life? Make a fivepoint quiz on it.

3. Mention Darwin’s two major contributions to evolutionary ideas.



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4. Write a note on NeoDarwinism.

5. List the five major events during geological time period beginning from origin of life.You may begin with origin of animals.

6. State major trends and stages of human evolution. Do you think humans are stillevolving? Write five sentences to justify your response.

7. Earlier groups of animals became extinct due to natural happenings. Today, how is it thatwild animals have become endangered and are heading towards extinction.

8. Write a ten sentences conversation between your father and yourself justifying theneed for conservation of animals living in our forest.

ANSWER TO INTEXT QUESTIONS

20.1

1. 4.5 to 5 billion years ago.

2. Too hot and only certain gases present and reducing atmosphere.

3. Remains of organisms who lived in the past.

4. The time 600 millions years ago when sudden formation of different groups ofinvertebrates on the earth took place.

5. (i) 144-65 mya (ii) 1.5-2 million years ago (mya) (iii) 57-34 mya

20.2

1. Founder of theory of evolution.

2. (i) all organisms related through ancestry

(ii) Mechanism of evolution is Natural Selection

3. Fittest individuals in a population survive and reproduce to leave the fit genes in thenext generation.

4. Darwin’s theory modified in the light of progress in genetics.

5. Natural Selection

20.3

1. 1.5 to 2 million years ago

2. Australopithecus

3. Homo sapiens

4. Apes

5. Australopithicus

20 HISTORY OF LIFE ON EARTH...our planet- earth such as (i) our universe (ii) our galaxy- milky way,...

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